1. Field of the Invention
The present invention relates generally to methods of measuring the concentration of an analyte in a fluid sample. More particularly, this invention provides methods for determining the appropriate point at which to take a measurement of an analyte concentration in a fluid sample applied to a test strip.
2. Background of the Invention
Monitoring analytes such as glucose, cholesterol, intoxicants, and other constituents is frequently desirable in fluids, such as blood, plasma, blood serum, saliva, urine, and other biological fluids. In healthcare applications, such monitoring affords the opportunity to make rapid diagnoses of a patient""s condition and to take prophylactic or therapeutic measures necessary for maintaining proper health.
One such healthcare application that has benefited tremendously by analyte monitoring in recent years is the treatment of diabetes. Diabetics suffer from an impaired ability to regulate glucose levels in their blood. As a result, diabetics can have abnormally high blood sugar levels known as hyperglycemia. Chronic hyperglycemia may lead to long-term complications such as cardiovascular disease and degeneration of the kidneys, retinas, blood vessels and the nervous system. To minimize the risk of such long term complications, diabetics must strictly monitor and manage their blood glucose levels.
Diabetics that have glucose levels that fluctuate several times throughout the day require very close blood glucose level monitoring. Close monitoring of blood glucose levels is most easily obtained when a diabetic is able to monitor their glucose levels themselves. Many devices currently available allow diabetics to measure their own blood sugar levels.
Reflectance-based monitors comprise one category of personal, or home-use, glucose level monitoring devices. These monitors utilize an optical block which accepts test elements for photometric analysis.
The test elements are usually in the form of test strips, which contain analytical chemistry. Conventionally, these test strips are in the form of a disposable diagnostic test strip containing analytical chemistry upon which a fluid sample is deposited. Once the user applies the fluid sample to the test strip, and the sample has sufficiently penetrated the test strip, a chemical reaction occurs in the presence of a target analyte, e.g., glucose, to cause a change in the optical properties of the test strip. An optical photometric device then determines the analyte level of the sample by measuring an optical property, such as the intensity of reflected light at a certain wavelength from the test strip. For in vitro analysis in healthcare applications, the fluid sample is usually fresh whole blood. Periodically, however, it is desirable to run a test on a test element formed by applying a control solution of known analyte concentration to a test strip, in order to verify that the meter is performing within operational limits. It is also desirable for the user to insert a xe2x80x9cstandard stripxe2x80x9d, which is a test element that has known optical properties, in order to verify that the meter is operating within operational limits.
Diagnostic test strips for testing analytes such as glucose levels of blood samples are well known in the art and comprise various structures and materials. Test strips typically include single or multi-layered porous membrane arrangements which receive a blood sample and undergo a change in an optical property, such as a color change, in response to the interaction of blood glucose with agents/reactants in the membrane. Examples of such multi-layer strips are described in U.S. Pat. No. 5,296,192 to Carroll and U.S. Pat. No. 6,010,999 to Carroll et al., the contents of both of which are incorporated herein by reference.
Prior to reaching the reactants, a whole blood sample can be filtered to eliminate potential optical interference by removing erythrocytes, or red blood cells. Some test strips operate to allow the applied blood sample to migrate to a reaction site in the membrane where the sample reacts with the agents/reactants, which is located in downstream capillary relation to the sample application site. The results of the reaction are often visible as a color change at the reaction site. However, the change may occur in invisible regions of the electromagnetic spectrum, such as infrared and ultraviolet. For the purposes of this application, the term xe2x80x9ccolor changexe2x80x9d will be understood to include variations in optical properties throughout the visible and invisible regions of the electromagnetic spectrum. As noted above, a color change can be correlated to the amount of glucose in the sample. Home-use glucose measuring devices that use a reflectance meter to measure the color change of the test strip correlate glucose levels to the change in the amount of light reflected from the reaction site of the test strip. As is well known in the art, strips can be formulated to produce a color change within a certain spectral region, and the meter designed to photometrically measure reflected, absorbed or transmitted light at a wavelength sensitive to the color change of the strip. While the present invention will be described with reference to reflectance based photometry, it would be known to one having ordinary skill in the art to apply the features of the invention to absorbance or transmittance based systems.
An important aspect to the accurate measurement of glucose levels in a fluid using a test strip are the methods used to calculate the glucose concentration values from the reflectance values obtained. Because different samples physically vary and will contain different levels of analyte, reaction rates and durations will vary. Prior art devices have focused on fixing an initiation point, the time from which the monitoring device begins to measure the chemical reaction of the blood sample with the test strip. This initiation point often was carefully tied to the initial contact of analyte and reagent, either manually or automatically and then the reaction was timed for a fixed period of time from this initiation point. The end point is the time at which the monitoring device takes a final reflectance reading to calculate the reported glucose level of the sample from calibration data stored in the meter""s memory. Because a fixed time (or times) is used in the prior art, calibration is simplified, but this approach requires waiting for a fixed period. The fixed time period is usually longer than required for the reaction to complete, resulting in user inconvenience.
Some home-use glucose monitoring devices have an initiation point corresponding to manual, or user determined events. For example, some monitoring devices trigger the initiation point for measuring glucose levels upon the pressing of a button, the insertion of a test strip into the monitoring device, or upon closing an element, such as a cover or door, of the monitoring device over the test strip. These user-defined initiation points decrease the accuracy and consistency of the monitoring device because they rely on the inconsistent timing of an action by the user (i.e. insertion or covering of the test strip in the monitoring device). These inaccuracies in determining the initiation point are commonly carried through to the end point measurement time. This results from the fact that many common monitoring devices use a fixed time period from the initiation point to determine when to initiate the end point measurement. This fixed period timing is especially problematic when using multi-layer test strips because of the nonuniform absorption periods inherent with such test strips, owing to physical differences between various samples (e.g. hematocrit, sample viscosity as well as general operational conditions such as temperature, humidity, etc.).
Accordingly, conventional methods for determining initiation and end points for measuring glucose levels from test strips may yield inaccurate results because the methods depend on events or time periods unrelated to the reaction kinetics of the blood sample and the test strip. Further, because reactions each occur at different rates, reliance on a fixed time period between an initiation point and the end point may prolong the measuring time beyond the time necessary to accurately measure the glucose level of the sample, resulting in user inconvenience. While a more accurate measuring time may only yield a few seconds improvement over a fixed measuring time, such an improvement is substantial to the person who uses a device multiple times daily. Further, a more accurate, uncomplicated and quick method for testing blood samples for glucose levels will encourage patients to monitor their blood sugar levels more regularly, thereby promoting compliance with their prescribed regimens for diabetes management.
It is accordingly a primary object of the invention to provide a method for rapid and accurate methods for determining an analyte concentration in a fluid sample.
In accordance with the invention, a method is disclosed for measuring an analyte in a fluid sample in a meter system, comprising the steps of: a) initiating a measuring period for a fluid sample; b) measuring a property of the fluid sample at each of n points in time, said multiple points in time being spaced at predetermined intervals to form a sequence of n measurement values, where n is an integer; c) comparing two nonconsecutive measurement values to determine at least one ratio; d) terminating the measuring of the fluid sample when at least one of said at least one ratio meets predetermined criteria indicating a final analyte value can be accurately determined; and e) determining a final analyte value based on said measurement values.
In further accordance with the invention, a meter system is disclosed for measuring an analyte in a fluid sample, comprising: a light intensity control circuit for measuring reflectance values of an analytic test strip onto which a fluid sample has been applied; and a processor for (a) initiating a measuring period for a fluid sample; (b) measuring the reflectance of a portion of the test strip onto which the fluid sample has been applied at each of n points in time, said multiple points in time being spaced at predetermined intervals to form a sequence of n calculated measurement values, where n is an integer; (c) comparing two nonconsecutive measurement values to determine at least one ratio; (d) terminating the reflectance measuring when at least one of said at least one ratio meets predetermined criteria indicating a final analyte value can be accurately determined; and (e) determining a final analyte value based on said measurement values; and a display for reporting said final analyte value in a user comprehensible format.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.